The present invention relates generally to nitride based semiconductor structures, and more particularly to a method of fabricating GaN substrates from an etched striped GaN layer grown on a sapphire substrate.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V semiconductors, comprise elements selected from group III, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as aluminum gallium indium nitride (AlGaInN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the near ultraviolet spectrum.
The shorter wavelength blue of nitride based semiconductor laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for high-resolution or high-speed laser printing operations and high density optical storage.
The primary impediment to the use of GaN as a laser structure substrate is GaN""s thermal decomposition at relatively low temperatures to produce metallic Ga and N2 gas. As a result, large area, freestanding GaN substrates are difficult to fabricate for nitride based semiconductor structures.
The conventional substrate material for semiconductor structures would be silicon or gallium arsenide. However, the GaN crystal structure, combined with the high GaN growth temperatures, make deposition of high-quality nitride semiconductor material directly onto semiconductor substrates such as Si or GaAs very difficult.
Nitride based semiconductor structures currently require heteroepitaxial growth of GaN thin layers onto dissimilar substrates such as sapphire or silicon carbide.
The most commonly used growth substrate, sapphire, still imposes constraints on the GaN layer quality due to the lattice and thermal-expansion coefficient mismatch between the GaN and the sapphire. The disparate properties of these two materials result in a high density of extended defects, such as dislocations and stacking faults, at the GaN thin layer/sapphire substrate interface.
Large-area (five centimeter diameter) GaN substrates can be fabricated from heteroepitaxial growth on various substrates such as sapphire or GaAs. The typical procedure involves deposition of a relatively thick GaN layer (greater than 80 xcexcm) onto the growth substrate followed by removal of the substrate, creating a freestanding GaN substrate for the nitride based semiconductor laser diode structure.
Many substrate separation techniques are available including wet-chemical etching, chemical-mechanical polishing or laser-assisted lift-off. Wet-chemical etching and chemical-mechanical polishing are inherently slow processes that require high selectivity in materials in order to remove the original growth substrate. Laser assisted lift-off processes have several advantages over the chemically assisted methods for the GaN thin film/sapphire substrate system. The laser processing is optically selective, possesses spatial control and is a relatively fast lift-off technique.
In order for the substrate separation technique to be successfully implemented, the technique itself must not degrade the quality of the GaN layer being processed. The laser process introduces a thermoelastic stress to the GaN layer, due to the rapid heating and cooling during the pulsed irradiation, that may fracture the GaN layer. Thin film fracture may arise from microcracks within the biaxially stressed GaN or from a thermal shock initiating microcrack propagation through the GaN layer.
An inherent problem when depositing thick GaN layers heteroepitixally onto sapphire or GaAs is the intrinsic stress, compressive for sapphire and tensile for GaAs, regardless of the substrate separation technique, due to the thermal coefficient mismatch between the GaN film and the substrates.
The success of the growth substrate removal to create a GaN substrate is dictated, in part, by the quality of the as-grown GaN layer. Due to complications related to heteroepitaxy, thick GaN layers, like those needed for a substrate, generally possess microcracks that can propagate and multiply during the laser lift-off process. The combination of the intrinsic residual stress and the thermoelastic stress of the laser processing gives rise to crack propagation across the entire GaN wafer area. The crack propagation would lead to uncontrolled catastrophic mechanical failure of the GaN or, at least, ill-defined low-quality GaN substrates.
Insulating substrates allow the economical construction of nitride based semiconductor lasers and laser diode arrays. Currently, nitride based single laser structures are grown on insulating sapphire substrates. The use of insulating substrates for laser diode arrays presents a special problem in providing electrical contacts for the laser diodes. In contrast to the situation where conducting substrates are used, insulating substrates cannot provide a common backside contact for a nitride based semiconductor structure. Hence, providing electrical contacts to laser diodes on insulating substrates has required the use of both contacts on the same side of the nitride based laser diode structure.
It is an object of the present invention to provide gallium nitride substrates from a trench patterned gallium nitride layer of a sapphire substrate.
According to the present invention, a gallium nitride layer is grown initially on a sapphire substrate. A mask layer, such as photoresist, a metal layer or a dielectric layer, is patterned into stripes on the gallium nitride layer. The gallium nitride layer is then etched down to the sapphire substrate to form trenches creating sectioned areas of gallium nitride on the sapphire substrate. Alternatively, trenches can be obtained by selective-area regrowth on a pre-patterned gallium nitride layer on the sapphire substrate.
The gallium nitride substrates are bonded to a silicon support substrate. Freestanding gallium nitride substrates are then fabricated using a laser lift-off process to remove the sapphire substrate and a solvent to remove the support substrate.
The edges of the gallium nitride substrates along the trenches act as terminating surfaces for cracks and defects, which originate during the growth of the gallium nitride layer, or that may propagate during the separation of the gallium nitride from the sapphire growth substrate.